Fluid-operated clutches (e.g., clutches operated by hydraulic or synthetic oil or other pressurized fluid) are generally well known and can be found in many systems and devices. Such clutches are hereinafter referred to as “hydraulic clutches.” One primary use for the hydraulic clutch is to provide shifting between differing input/output gear ratios within a power transmission. Typically, a transmission includes an input shaft and an output shaft, as well as one or more trains of interrelated gear elements usable to selectively couple the input and output shafts. The selection of a gear ratio at the output shaft is executed via one or more clutches that affect the rotations and/or interrelationships of the gear elements. The clutches are typically hydraulically driven to engage band or disk torque transfer elements.
Shifting from one gear ratio to another normally involves releasing or disengaging an off-going clutch or clutches associated with the current gear ratio and applying or engaging an oncoming clutch or clutches associated with the desired gear ratio. Although many different clutch arrangements are possible within such transmissions, an arrangement that provides the greatest simplicity is a two-clutch shifting transmission. In this arrangement, only two clutches are actuated for a given shift regardless of the number of clutches in the transmission. In other words, a shift is executed by deactivating a single “off-going” clutch and activating a single “oncoming” clutch.
Each hydraulic clutch is typically controlled via an electrically controlled solenoid valve. The solenoid valves are electrically modulated to control hydraulic fluid pressure to the clutch and hence to control the clutch movement (e.g., in the absence of contact) and pressure (e.g., during contact).
In general terms, the clutches within a transmission are controlled both with respect to the engagement force of individual clutches as well as the timing or phase between clutch activation, e.g., between dropping an off-going clutch and activating an oncoming clutch. The force and phase with which the transmission clutches are manipulated greatly impact the resulting shift quality. For example, if the off-going clutch disengages prematurely, the engine speed may surge momentarily before the oncoming clutch begins torque transfer, resulting in a rough shift. Similarly, if the oncoming clutch engages prematurely, a suboptimal shift can result. In addition to creating an unpleasant user experience, poorly executed shifting can also impact the efficiency and service life of the transmission. To this end, it is desirable to calibrate the clutches of a hydraulic transmission.
One known method for calibrating transmission clutches is shown in U.S. Pat. No. 5,737,979 to McKenzie et. al entitled “Method of Calibrating Clutches in a Transmission.” This and similar methods for calibrating transmission clutches require that the machine remain immobile for an extended period of time. During this idle time, an automated process cycles each of the various clutches through a sequence of steps, checks their operation, and adjusts clutch parameters. Such calibration processes therefore delay the deployment of the machine and consume operator time supervising the machine during the calibration.
Accordingly, there is a need for a transmission clutch control system that provides effective, convenient, and unobtrusive calibration during normal operation of a machine in order to generally enhance transmission usability and longevity.